Thousands of San Francisco property owners would have to dig deep into their pockets to pay for mandatory earthquake retrofits of their buildings under a plan Mayor Gavin Newsom said he supported Thursday.

A recent city-sponsored report recommended mandatory retrofits for about 2,800 large, wood-frame buildings that are liable to collapse or sustain serious damage in a major quake centered near San Francisco.

Such a temblor, which could be as big as the 1906 quake that devastated the city, is likely to hit before 2032, according to the report.

The total cost to shore up the largest so-called soft-story buildings, which are believed to be the city's most vulnerable, would be about $260 million, but about $1.5 billion in possible damage could be prevented. For building owners, the cost could range from $9,000 to $28,000 per residential unit.

Newsom had supported voluntary measures to encourage retrofitting. But Thursday, he said that engineers and other experts analyzing the issue had come to the conclusion that "mandatory is necessary."

"That needs to be the framework of discussion now," he said at gubernatorial campaign stop in Stockton. "We might as well admit to that as the end result. We need to let folks know" that mandatory retrofits are the intent.

Property owners in San Francisco said that they would need financial assistance from the city, especially given the current economic climate.

"Our primary goal is to make our tenants safe, and the mayor's intentions are good and our intentions are good, but financing is the third leg on the stool on this issue," said Janan New, executive director of the San Francisco Apartment Association, which represents 3,000 small and large apartment building owners.

New said businesses and residents would be displaced by retrofit construction work and that landlords are required to pay hefty relocation fees.

Newsom would not say when legislation would be introduced at the Board of Supervisors, and he noted that he did not expect the retrofitting to be done all at once.

"There needs to be a process and a timeline that addresses the financial concerns, particularly in this economic crisis. Not everyone can afford to retrofit their building, we know that. We want to phase this in," he said.

Space without walls

The soft-story structures at issue are the classic San Francisco apartment buildings with a store or restaurant on the first floor. They get their name from a ground-floor space - a window or garage door - situated where a wall might otherwise be.

San Francisco has more such buildings than any other Bay Area city, and the buildings are more precarious in neighborhoods perched on unstable soil.

The open space sitting below several floors makes the frames prone to twisting and buckling, and many such buildings were damaged in the Marina district in the 1989 Loma Prieta earthquake and in Southern California during the 1994 Northridge earthquake.

Newsom said in July that he did not believe it was necessary to require owners to shore up their buildings as other Bay Area cities had done. But in recent months, he has said he might change his mind after reviewing more data.

Preventive action

In recent weeks, San Francisco Planning + Urban Research Association, a leading local think tank, also urged the city to require building owners to strengthen soft-story buildings and projected bleak conditions that San Francisco might face - including mass displacement of residents - after an earthquake if the city did not demand retrofits and take other preventive action.

The buildings that would be the subject of a retrofit requirement house nearly 60,000 residents and 7,000 employees but represent only a fraction of the structures that might be destroyed in the city if a big temblor were to hit today.

The large soft-story buildings studied comprise only 10 percent of the city's residential units that are believed to be unsafe. There are thousands of shorter soft-story buildings and others with fewer units that also might not hold up in a quake. Large concrete buildings lacking sufficient steel in their columns and beams also are a concern.

Earthquake consultants are scheduled to study more building types in the coming months and report back to the city about their vulnerability.

Chronicle reports last summer highlighted the fact that the city had no strategy for fixing the soft-story problem even though the danger had been known for decades.

At that time, as part of a study that had recently been restarted by the Department of Building Inspection, Newsom directed city employees and earthquake consultants to first analyze soft-story structures and to develop retrofit guidelines for them by the end of last month.

A 10-year plan

While the final study has yet to be released, a draft report calls for mandatory retrofits within 10 years. The report also recommends that that the repairs ensure that buildings would not only make it through a large quake, but also be habitable immediately afterward.

Laura Samant, a seismic engineering consultant who has led the city's studies, said she was glad to hear that Newsom was on board with a required retrofit program.

"We have recommended a mandatory retrofit ordinance for the city because we have decades of experience showing that these buildings don't get retrofitted if you don't mandate it," Samant said. "These are very dangerous buildings and that's why we have recommended the mandate."

The report does not provide details about the specific codes that should guide the retrofit work or what materials would be used. According to Samant, those details would be hashed out later by committees of engineers working with the city.